1914] Little: Ocelli of Pohjorchis 313 



from the base of the lens. Viewed from above, the lens body 

 cannot be distinguished, as only a round mass of pigment is 

 apparent. 



A radial section through the lower part of the bell and ten- 

 tacle (pi. 15, figs. 4 and 5) shows the general arrangement of the 

 eye-spot with reference to the surrounding parts of the bell. 

 The circular canal {cir. can.) is seen in cross-section. Extending 

 from it is the branch canal {tent, can.) into the tentacle which is 

 attached for a part of its length to the base of the bell. The 

 main part of the bell consists, on the external side, of a thin 

 epithelial layer of low cohunnar cells of the outer ectoderm 

 (o. ect.). At the base of this layer, lying along the basal mem- 

 brane (bas.m.) are longitudinal {l.mns.fib., pi. 14, fig. 4) and 

 circular {circ. mus. fib.) muscle fibers which extend all over the 

 surface of the outer part of the bell, thus making a broad, mus- 

 cular layer underneath the epidermis. On the inner .side of the 

 bell (pi. 14, fig. 5) is a similar epidermal layer underlaid by a 

 circular muscular layer {circ. mus. fib.). Between these two ex- 

 ternal layers of epidermis lies the transparent non-cellular 

 mesogloea {mes.) which forms the jelly-like contents of the live 

 animal. Embedded in or extending through the matrix near the 

 inner wall of the bell is what at first appears to be a thin layer 

 of cells similar to those of the outer surface. Close inspection 

 reveals it to be composed, not of a single, but a double layer of 

 more or less compressed cells, the entoderm {ent.). The basal 

 membrane at the base of this double row of cells extends down 

 to the ring canal {cir. can.), where it turns, forming the basal 

 membrane of the tentacle, the eye, and the outer epithelium of 

 the exumbrella. The connective tissue at the base of the sub- 

 umbrellar epithelial layer extends down aroimd the inner part 

 of the ring canal through the length of the velum. 



At the base of the velum near the ring canal the epithelium 

 becomes very much thickened, being more so on the lower llian 

 the upper side (pi. 14, fig. 5, n. r., 2 and 1) in two nerve rings. 

 These will be discussed later. 



The epithelium continues along both faces of the velum as a 

 thin layer of cells, but the ectodermal structure of the tentacle 

 becomes very much modified and specialized. This gradually 



